Grain size of fine-grained windblown sediment: A powerful proxy for process identification

Dust transport by the wind is not a uniform process but may occur in different modes according to source area conditions and transport height and distance. Subsequently, these differences are expressed in terms of grain-size and fluxes of the aeolian deposits. Transport distances may vary from several tens of meters to thousands of kilometers, while the height accordingly may vary from meters to more than thousand meters. A relation with grain-size distributions may be established on the base of type occurrences of different loess facies. Three main loess populations (and several subpopulations) of primary windblown origin may be defined according to their grain size (dominated by fine sand to very coarse silt, silt and very fine silt to clay, respectively). Each of them reflects a specific aeolian process and transport conditions. It follows that the grain-size distribution of a loess deposit is an excellent proxy for the reconstruction of aeolian processes and wind circulation patterns.Apart from (primary) pure wind deposition loess may also be affected by (secondary) post-depositional processes. Examples are settling of loess particles in a lacustrine setting and reworking by rivers or surface runoff. Although the primary loess characteristics are maintained, reworking processes leave also their imprint in the grain-size distribution as a useful tool for secondary process identification.